This invention relates broadly to automatic position-control valve assemblies and more specifically to safety features for them.
It is well known that factories and other industrial installations are increasingly automated. There are, for example, apparatus for automatically and continuously repositioning industrial valves to achieve desired fluid flows. Often desired flows change with changing conditions. For example, it may be desirable to control fluid flow so as to maintain a selected pressure, temperature, flow rate, or level. In fact, many industrial installations include fluid-condition transducers for monitoring flow (as measured by temperature, pressure, flow rate, and so forth), with electrical signals. Similarly, many industrial valves include valve position feedback potentiometers, or transducers, sometimes combined with limit switches, for providing signals which are indicative of valve positions. Normally, a valve position signal is used to provide a visual readout as to a valve""s position. In some cases a fluid-condition signal is fed to a set point process controller which determines therefrom a new desired position of an industrial valve element. The set point process controller then sends a xe2x80x9cdesired-position signalxe2x80x9d to a valve position controller which adjusts an industrial valve element to a new position by means of a pressurized-fluid system (such as a pneumatic system). Changing the position of the industrial valve element causes a desired change in the condition of the fluid.
Prior-art systems include various components of this apparatus in single housings, while others separate the components in other ways. U.S. Pat. No. 4,417,312 to Cronin et al., U.S. Pat. No. 5,198,973 to Steurtermann and U.S. Pat. No. 5,654,885 to Mayhew et al. disclose various prior-art systems of this general type.
In many such prior-art systems a valve position transducerxe2x80x94such as a Moore Valve PAC(trademark) Digital Valve Controller, Intelligent Valve Control, series 760Dxe2x80x94is rigidly attached to a valve position actuator, so that the valve position transducer can sense a position into which the valve position actuator has moved the valve element via the valve position actuator. An actuating fluid, such as air, is fed to the valve position transducer. The valve position transducer compares a desired-position signal with the position of the actual valve element and from this comparison determines a direction of movement for the valve element. To achieve valve-element movement, the valve position transducer feeds actuating fluid to the valve position actuator to move the valve element to the new desired position.
A difficulty that sometimes develops in use of such automatic position-control valve assemblies occurs when a valve position transducers fail. In this regard, valve position transducers require extremely clean air, which is difficult to maintain in some industries. Such valve position transducers, for example, will not work with regular compressed air. Users must normally work on such valve positioners regularly to keep them clean. In any event, they sometimes have control signal problems. When valve position transducers have such problems, or otherwise fail, they sometimes drive valve elements to incorrect positions, with drastic results. When this has happened, or when valve position transducers have been worked on, it has been necessary for operators to try to move the valve elements to appropriate positions by hand or to quickly replace the valve position transducers, or replace the entire valve assemblies, including the valve position transducers and the valve position actuators. Usually, these corrective steps have required that the monitored-and-controlled fluid flow be turned off until corrective actions have been taken, thus, the entire operations have been shut down. Further, it has often been inconvenient to replace such components, because replacement parts have not always been immediately available and/or technicians with the ability to replace them have not been available. Thus, operations have often been shut down for long periods of time.
Thus, it is an object of this invention to provide an override mechanism for easily moving a valve element of a valve controlled by a valve position transducer to a desired position when its valve position transducer malfunctions, while allowing the valve position transducer to be replaced without shutting down a primary fluid flow being controlled.
According to principles of this invention, an override controller is structured for being placed in pressurized fluid lines between a valve position transducer and a valve position actuator of a automatic position-controlled valve assembly, with the override controller receiving uncontrolled pressurized fluid independently of the valve position transducer. The override controller includes a valve mechanism for receiving controlled pressurized fluid at first and second inlet ports from the valve position transducer and furnishing this pressurized fluid to the valve position actuator through first and second outlet ports for controlling the position of a valve element during normal operation. However, the valve mechanism of the override controller can also be set for stopping flow of controlled pressurized fluid through the first and second inlet ports and developing itself controlled fluid flow from the uncontrolled pressurized fluid to the first and second outlet ports for controlling the position of the valve element during an override operation.